Optical bistable devices are capable of achieving two different stable optical states, such as transmitting and blocking. They can modulate a reference light source to perform optical switching and logic functions and are useful for optical communications in a variety of ways.
One particularly useful optical bistable device includes semiconductor nonlinear absorber material which is in a Fabry-Perot interferometer cavity configuration. A reference light source beam is incident on an input end face of the cavity. The output transmitted from the other, output end face of the cavity is modulated by a switching light source, which may be incident on the input face together with the reference beam or from some other direction. An increased intensity of light in the cavity saturates the absorber to thereby decrease its absorption coefficient. This permits the light intensity within the cavity to exceed a threshold level sufficient for cavity resonance and a transmitting of the reference beam. A sufficient decrease of the light intensity within the cavity unsaturates the absorber again. This brings the cavity back out of resonance to result in a return to its initial blocking state. The process of switching states has an optical hysteresis, due to the fact that the cavity has a tendency to remain in resonance.
Various optical bistable devices and their mode of operation have been described in a number of patents and publications, such as, for example:
1. "Integrated Bistable Optical Devices", Appl. Phys. Lett., Vol. 33, No. 1, July 1978, by P. W. Smith, I. P. Kaminow, P. J. Maloney, and L. W. Stulz, pp. 24-27; PA1 2. "Polarization-Independent Optical Directional Coupler Switching Using Weighted Coupling", Optical Communication Conference Proceedings, Amsterdam, Sept. 17-19, 1979, by R. C. Alferness, pp. 11.4-2-11.4-4; PA1 3. "Optical Modulation by Optical Tuning of a Cavity", Appl. Phys. Lett., Vol. 34, No. 8, April 1979, by H. M. Gibbs, T. N. C. Venkatesan, S. L. McCall, A. Passner, A. C. Gossard, and W. Wiegmann, pp. 511-514; PA1 4. "Optical Bistability and Modulation in Semiconductor Etalons", Topical Meeting on Integrated and Guided-Wave Optics, A digest of technical papers presented at the Topical Meeting on Integrating and Guided Wave Optics, Jan. 28-30, 1980, by H. M. Gibbs, S. L. McCall, T. N. C. Venkatesan, A. Passner, A. C. Gossard, and W. Wiegmann, pp. MB5-1-MB5-4; PA1 5. "A Bistable Fabry-Perot Resonator", Appl. Phys. Lett., Vol. 30, No. 6, March 1977, P. W. Smith and E. H. Turner, pp. 280-281; PA1 6. U.S. Pat. No. 4,012,699 issued Mar. 15, 1977 to H. M. Gibbs, S. L. McCall, and T. N. C. Venkatesan; and PA1 7. U.S. Pat. No. 4,121,167 issued Oct. 17, 1978 to H. M. Gibbs, S. L. McCall, and T. N. C. Venkatesan.
It is known to construct a bistable device which uses phase modulation and has no Fabry-Perot cavity, provided there is a negative feedback mechanism. Such an approach is described, for example, in the publication 5 above.
One problem with present approaches to the above-described optical bistable devices is that the nonlinearity of the absorber depends on the photogeneration by the switching light of a relatively large population of conduction band electrons in the absorber. At low temperatures this photogeneration can be achieved with a light of reasonable intensity, particularly using exitons with energies close to the bandgap of the semiconductor. At room temperature, however, the lifetime of the conduction band electrons is so short that a very intense switching light is required in order to generate the conduction band electron density needed for bistability, thus resulting in a device with an unacceptably low sensitivity. The requirements for a low temperature or for a high intensity switching light both lead to relatively complex structures and relatively high power requirements for the bistable device. Therefore, they preferably are avoided for the integrated optics devices for optical communication systems.